The Mark II was constructed with high-speed electromagnetic relays instead of electro-mechanical counters used in the Mark I, making it much faster than its predecessor. Its addition time was 125,000 microseconds and the multiplication time was 750,000 microseconds (this was a factor of 2.6 and a factor 8 faster, respectively, compared to the Mark I). It was the second machine (after the Bell Labs Relay Calculator) to have floating-point hardware. A unique feature of the Mark II is that it had built-in hardware for several functions such as the reciprocal, square root, logarithm, exponential, and some trigonometric functions. These took between five and twelve seconds to execute.

The Mark II was not a stored-program computer – it read an instruction of the program one at a time from a tape and executed it (like the Mark I). This separation of data and instructions is known as the Harvard architecture. The Mark II had a peculiar programming method that was devised to ensure that the contents of a register were available when needed. The tape containing the program could encode only eight instructions, so what a particular instruction code meant depended on when it was executed. Each second was divided up into several periods, and a coded instruction could mean different things in...Read More